1. Field of the Invention
The present invention relates to a method for controlling an oxygen concentration detection apparatus, and more specifically to a method for controlling the supply of heater current to a heater element provided in the oxygen concentration detection apparatus.
1 Description of Background Information
In order to accelerate the purification of the exhaust gas and to improve the fuel economy of an internal combustion engine, a feedback type air/fuel ratio control system is generally used, in which oxygen concentration in the exhaust gas is detected and air/fuel ratio of the mixture supplied to the engine is controlled to a target air/fuel ratio by a feedback control operation in accordance with a result of the detection of the oxygen concentration.
As an oxygen concentration detection apparatus for use in such an air/fuel ratio control system, there is a type which is capable of producing an output signal whose level is proportional to the oxygen concentration in the exhaust gas of the engine in a region in which the air/fuel ratio of the mixture is larger than a stoichiometric air/fuel ratio, and the detail of which is disclosed in Japanese patent application laid open No. 58-153155. This oxygen concentration detection apparatus includes an oxygen concentration sensing unit having a general construction including a pair of flat solid electrolyte members having oxygen ion permeability. These oxygen ion conductive solid electrolyte members are placed in the exhaust gas of the engine, and electrodes are respectively provided on the front and back surfaces of both of the solid elctrolyte members. In other words, each pair of electrodes sandwich each solid electrolyte member. These two solid electrolyte members each having a pair of electrodes are arranged in parallel so as to face each other and forming a gap portion, or in other words, a restricted region between them.
With this arrangement, one of the solid electrolyte members serves as an oxygen pump element and the other one of the solid electrolyte members serves as a sensor cell element for sensing an oxygen concentration ratio. In an ambient atmosphere of the exhaust gas, a drive current is supplied across the electrodes of the oxygen pump element in such a manner that the electrodes facing the gap portion operates as a negative electrode. By the supply of this current, i.e. a pump current, the oxygen component of the gas in the gap portion is ionized on the surface of the negative electrode of the oxygen pump element. The oxygen ions migrate through the inside of the oxygen pump element to the positive electrode, where the oxygen ions are released from the surface thereof in the form of the oxygen gas.
While this movement of the oxygen ions is taking place, the oxygen concentration becomes different for the gas in the gap portion and the gas outside the sensor cell element because of a decrease of the oxygen gas component in the gap portion. Therefore, a voltage whose magnitude varies substantially linearly in proportion to the oxygen concentration of the gas to be measured is generated across the electrodes of the solid electrolyte member operating as the sensor cell element, if the magnitude of the electric current supplied to the oxygen pump element, i.e the pump current, is constant.
By means of this voltage generated across the electrodes of the sensor cell element, a detection as to whether the air/fuel ratio of the mixture supplied to the engine is rich or lean is performed. In the case of the air/fuel ratio control system in which the air/fuel ratio is controlled by the supply of the air intake side secondary air, the secondary air is supplied when the air/fuel ratio is detected to be rich. On the other hand, the supply of the secondary air is stopped when the air/fuel ratio is detected to be lean, and the air/fuel ratio is controlled toward a target air/fuel ratio by the supply and stop of the air intake side secondary air. Further, if the magnitude of the pump current supplied to the oxygen pump element is varied so that the voltage developing across the electrodes of the sensor cell element becomes constant, the magnitude of the pump current varies substantially in proportion to the oxygen concentration in the exhaust gas, under a condition of a constant temperature.
In this type of the oxygen concentration sensing unit, it is necessary that the temperature of the sensing unit is sufficiently high. Especially in the case of the above mentioned proportional type oxygen concentration sensing unit, its operating temperature must be higher (for example, higher than 650.degree. C.) than an exhaust gas temperature under a steady state operation, in order to obtain a proportional output signal characterisic in which the sensor output signal varies substantially in proportion to the oxygen concentration. To meet this requirement, a heater element which is made up of a heater wire, for example, is incorporated in the oxygen concentration sensing unit and a drive current is started to be supplied to the heater element upon starting of the engine operation so that heat is generated at the heater element.
As the heater element, it is general to use a material having a positive resistance temperature coefficient, such as a nickel-chromium wire. This means that, upon cold start of the engine, the internal resistance of the heater element is smaller than the value under a hot start of the engine. Therefore, when the supply of the heater current is started when the engine is cold, an excessive rush current flows through the heater element as typically illustrated in FIG. 1. By this excessive current, a rapid deterioration of the heater element or a wire break in the heater element is induced. Thus, there has been a problem that the longevity of the heater element is rather short. Further, it has been possible that a breakdown of the oxygen concentration detection unit occurs due to its rapid temperature rise immediately after the start of the supply of the heater current.